I have to agree with MMACH5 on the sound issue. Sound is very much a BAD indicator of a car's presence.

All too often the wind noise in my own ears makes an approaching car totally undetectable other than seeing the car in my helmet mirror. Also there are places where I'm on a small road with a nearby parallel interstate, and the noise of traffic on the interstate sounds just like a car coming behind me, but then no one will be there.

This is pretty much the exact reason why I started using a helmet mirror. I simply could not rely on sound to tell when a car was coming behind me, and I don't want to be turning back to look every few seconds.